Process for strengthening alloys

ABSTRACT

A process is provided for strengthening stainless steel and titanium alloys by explosive shocking of the alloys at cryogenic temperatures while substantially prohibiting macroscopic deformation. The alloys are preferably subjected to shock wave pressures of 225 to 275 kb. at temperatures of -100* to -200* F.

United States atent 72 Inventor Eugene J. Delgrosso 2864,69775/11138' 13 usch etal. 75/1755' Wallingford, Conn. 3,215,565 11/1965 Harvey 148/12.4 [21] App1.No. 777,381 3,329,535 7/1967 Langer et al 148/4 [22] Fil d N v.20, 1968 3,454,435 7/1969 Jacobs 148/12.7 Patented Oct. 26,1971 FOREIGN PATENTS [731 Assign fi t fi z 153,476 1963 U.S.S.R. 72/56 as a OTHER REFERENCES Trans. of Aime, Vol. 227 Oct. 1963, pages 1,188 1,193 PROCESS FOR STRENGTHENING ALLOYS USAtl? COIISGEZCI No. Af323(]6)186L)l1i191- ACI'OJCI General Cor- 18 Claims, 3 Drawing Figs. P p g Johnson et a1., Measurement of Dynamic Plastic Flow [52] US. Cl 148/125, Properties Under Uniform Stress," ASTM Special Tech, Pub,

148/4,148/127,148/131 336,1963, pages 195- 205 [51] Int. Cl C221 3/00, Primary Examiner charles N. Love C22f l/lsczld H0O AtlomeyFinneg an Henderson & Farabow Field of Search 148/4, 12,

ABSTRACT: A process is provided for strengthening stainless [56] References Cited steel and titanium alloys by explosive shocking of the alloys at UNITED STATES PATENTS cryogenic temperatures while substantially prohibiting macro- 3,197,851 8/1965 Aleck 29/421 scopic deformation. The alloys are referably subjected to P 2,527,287 10/1950 Ziegler et a1 148/12.7 X shock wave pressures of 225 to 275 kb. at temperatures of 2,703,297 3/1955 MacLeod 148/4 to -200 F.

F. 7 'T IO IO 34 I 2 7 1 in L a l 1 a 24 a PATENTEnnm 26 197i 3.615.921

SHEET 1 OF 2 1 INVENTOR EUGENE J. DELGROSSO Zmcyan J l'n c/e/xsorz (xii 06 0a) ATTORNEYS PATENTEDUET 2 s [9H SHEET 2 (IF 2 PHOTOMICROGRAPHS OF ANNEALED AlSl 30l STAINLESS STEEL ALLOY SPECIMEN.

PHOTOMICROGRAPHS OF CRYOSHOCKED AlSl 30| STAINLESS STEEL ALLOY SPECIMEN.

INVENIOR EUGENE J. DELGROSSO Emegan, jfenc/mon (glfd lagoa) ATTORNEY 5 PROCESS FOR STRENGTHENING ALLOYS This invention relates to a process for strengthening metal alloys. More particularly, it relates to a process for strengthening stainless steel and titanium alloys by explosive shocking at cryogenic temperatures.

Increasing the strength properties of metals and alloys is, of course, very important to the metals industry. As more strength is imparted to a metal, it can serve a wider variety of functions, and can serve these functions in a more effective and economical manner.

Metals and alloys have been heretofore strengthened by a wide variety of procedures and processes. Most of these procedures involve some form of working of the metals and alloys.

Voluminous amounts of research have been carried out to develop new and more effective and efficient procedures for strengthening metals and alloys. This research has investigated the techniques and the equipment used in strengthening, and has evaluated which of these techniques and equipment can best be used in strengthening each of various types of metals and alloys.

Certain recently proposed metal strengthening processes involve the use of cryogenic temperatures. While these cryogenic strengthening processes have contributed advances to the art, they have also suffered from shortcomings. The prior art cryogenic strengthening processes have uniformly required macroscopic deformation of the metal or alloy being strengthened. Thus the product is typically reduced to a cryogenic temperature and rolled or stretched 5 percent or percent or even more to achieve the desired strengthening.

Attempts to practice such cryogenic stretching processes commercially raise significant problems in achieving the desired shape in the final product, in ensuring uniform deformation of all parts ofthe product, and in avoiding extreme and damaging losses of ductility in the product. Moreover, cryogenic stretching processes are generally expensive because ofthe high degree of control which must be exercised over the deformation to achieve the desired product.

Cryogenic stretching processes are usually also limited in their application to certain shapes and compositions of materials. Certain of these processes, for example, can be used only with metal vessels which can be filled with a cryogenic material, such as liquid nitrogen, and cannot be used to strengthen flat plates, castings, or the like. Other cryogenic stretching processes can be used only with special, low-carbon stainless steels and cannot be used with more conventional, higher carbon stainless steels.

It is accordingly a primary object of the present invention to provide an improved process for strengthening metal alloys.

Another object of this invention is to provide an improved process for strengthening metal alloys at cryogenic temperatures.

A further object is to provide an improved process for strengthening stainless steels and titanium alloys in the form of flat metal plates, castings, or articles of virtually any shape, in which the articles are strengthened at cryogenic temperatures by a procedure that avoids many of the inherent disadvantages of cryogenic stretching.

Additional objects and advantages of this invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the processes and methods particularly pointed out in the appended claims.

To achieve the foregoing objects, and in accordance with its purpose, this invention provides a process for strengthening metal alloys which comprises subjecting an alloy selected from stainless steels containing substantial amounts of metastable austenitic phase at room temperature, and titanium alloys consisting primarily of metastable beta-phase at room temperature to at least one explosive shock at a cryogenic temperature less than about -75 F. while substantially prohibiting macroscopic deformation of the alloy.

The invention consists of the novel methods, processes and improvements shown and described. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the practice of the invention, and, together with the description herein, serve to explain the principles of the invention. Of the drawings FIG. 1 is a schematic illustration of the practice of one embodiment of the cryogenic shocking process of this invention.

FIG. 2 shows photomicrographs of a specimen of annealed A181 301 stainless steel alloy prior to cryogenic shocking in accordance with the present process. In photomicrograph A the sample is magnified 50 times, and in photomicrograph B the sample is magnified 200 times.

FIG. 3 shows photomicrographs of a specimen of the AlSl 301 stainless steel alloy of FIG. 2 after it has been subjected to a single cryogenic explosive shock of 250 kb. at -l00 F., in accordance with the present process. The sample is magnified 50 times in photomicrograph A and 200 times in photomicrograph B.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory but are not restrictive of the invention. Reference will now be made in detail to the description of the presently preferred embodiments of the invention.

In accordance with the invention, a process is provided for strengthening stainless steel and titanium alloys at cryogenic temperatures. The alloys treated in accordance with this process are stainless steel alloys containing substantial amounts of metastable austenitic phase at room temperature, and titanium alloys consisting primarily of metastable betaphase at room temperature.

The stainless steel alloys that can be treated by the present process include commercially available steels of the 300 series, such as, for example, 301 stainless steel, 302 stainless steel, 304 stainless steel, and 3l0 stainless steel. These steels have the following nominal compositions:

(30]) Fe-l6 to l8 Cr6 to 8 Ni-up to 2 Mn-up to 1 Siup to 0.15 C

(302) Fel7 to 19 Cr8 to l0 Ni-up to 2 Mn-up to 1 Siup to 0.15 C

(304) Fe-l8 to 20 26 Crll to [2 Ni-up to 2 Mn-up to l Siup to 0.08 C

(310) Fe-24 to 26 Crl9 to 22 Ni-up to 2 Mn-up to L5 Siup to 0.25 C

The present process can also be utilized in strengthening precipitation hardening stainless steels that contain substantial amounts of metastable austenitic phase at room temperature. These precipitation hardening steels can be aged prior to shocking; or they can be in a solution heat-treated condition prior to shocking and be suitably aged after shocking to produce the desired precipitation. The aging treatment used should be one that does not cause transformation of the martensite phase formed by the shocking step.

Exemplary of the precipitation hardening stainless steels that can be treated by the present process are commercially available AM 362 stainless steel, which contains about 20 to 30 percent metastable austenite at room temperature, and 17-7 PH stainless steel. These steels have the following nominal compositions:

(AM 362) Fe-l5 Cr6.5 Ni-0.8 Ti-0.03 C (l7-7 PH) Fe.l7 Cr-7 Ni-l.l5 Al0.7 Mn-0.4 Si- 0.07 C.

It is to be understood that the present process is applicable not only to the specific stainless steel alloys listed above, but to all stainless steel alloys, including precipitation hardening alloys, which contain substantial amounts (at least 20 percent or greater for example) of metastable austenitic phase at room temperature. The presence of this metastable phase allows austenite to martensite transformation to occur during cryogenic shocking. This transformation is believed to be one of the essential factors producing the strengthening of this invention.

Titanium alloys which consist primarily of metastable betaphase at room temperature can also be strengthened in accordance with the present process. Alpha-phase titanium alloys (at room temperature) and Ti alloys which comprise mixtures of substantial amounts of alpha-phase with beta-phase at room temperature have a strong tendency to crack at cryogenic temperatures, and thus the present process is generally not useful in strengthening such alloys. However, beta-phase titanium alloys such as Ti-l to 16 V2.5 Al and Ti-- 13 V1 l Cr-3 Al, as well as the so-called beta-ill alloys, such as Ti-l 1.5 Mo-6 Zr4.5 Sn, are exemplary of the titanium alloys which can be significantly strengthened by the cryogenic shocking process of this invention. Such Ti-alloys should contain about 80 to 90 percent or more of metastable beta-phase at room temperature.

The alloy to be strengthened is contacted with a cryogenic material to reduce its temperature to at least about 75 F, before it is subjected to the explosive shocking of the present process. The cryogenic shocking step is usually carried out at temperatures between about 75 and 320 F. It is possible to use temperatures lower than 320 F, but to do so it is necessary to work with liquid helium or liquid hydrogen, and the equipment costs and cost of handling these materials are generally considered to render their use economically unfeasible.

The cryogenic shocking step of the present process is preferably carried out at temperatures between about l00 and 200 F., with about l00 F., being considered optimum. Liquid nitrogen is a preferred cryogenic material which can be used to reduce the alloys to be strengthened to these temperature levels. Freon-type liquids can be used as the cryogenic material for l00 F, operation and the lower expense incident to its use is one factor which causes -l00 F. to be considered an optimum operating temperature.

As pointed out above, the strengthening of the alloy is achieved in the present process by subjecting it to explosive shocking at these cryogenic temperatures. Through the use of proper explosive mechanisms, the amount of shock applied to the alloy can be controlled, and calibrated in advance. The alloy can be subjected to a single explosive shock or to a series of sequential shocks. It has been found that use of two sequential shocks provides optimum strengthening. If more than one shock is used, at least one, and preferably all, of the explosive shocks are applied to the alloy at the cryogenic temperatures described above.

The process of this invention can be practiced by explosively shocking the alloy twice, once at cryogenic temperatures, for example 100 to 200 F., and once at ambient or room temperature. Excellent, though not optimum, strengthening is achieved by this procedure, and it affords good process economy since no cryogenic material need be used in one of the shock steps.

In accordance with the process of this invention, the pressure applied to the alloy by each explosive shock is between about 150 and 450 kilobars (kb.) and preferably between about 225 and 270 kb. Use of shock pressures in excess of 450 kilobars can produce excessive instances of damage to or even fracturing of the alloy being strengthened. Use of shock pressures below about 150 kb. does not produce desired levels of strengthening and use of pressures of at least about 200 kb. is highly advantageous when substantial strengthening is desired. Thus, shock pressures on the order of 150 to 175 kb. should be used only when moderate strengthening is required and retention of high ductility is important.

In accordance with the most preferred practice of the present process, the alloy being strengthened is subjected to two sequential explosive shocks, each of which applies a pressure of about 225 to 275 kb. to the alloy at a temperature between about l00 and 200 F.

The explosive shock can be applied to the alloy specimen in accordance with the present process by well-known techniques. Thus, for example, a sheet of explosive material, such as pentaerythrito] tetranitrate or the like, can be applied to the surface of the alloy to be strengthened in a thickness dependent on the explosive pressure to be applied to the alloy.

The explosive, however, cannot be detonated in direct contact with the alloy, because such direct contact would likely cause deterioration of the metal surface of the alloy by surface checking, dimpling and sometimes even cracking. It is, therefore, necessary to insert a buffer plate between the workpiece and the explosive. This buffer plate may be metallic, but is is difficult to use metallic buffer plates with complex alloy shapes. For such shapes it is preferred to coat a surface of the alloy workpiece with a plastic or epoxy coating that serves as a buffer plate.

It should be noted that when such plastic buffer plates are used, their presence must be evaluated in determining the shock pressures to be applied through them to the alloy workpiece. Shock waves are transmitted differently through plastics than they are through metals, and it is necessary to apply a correction factor to determine the equivalent shock pressure applied using plastic and metal bufiers.

In accordance with the process of this invention, macroscopic deformation of the alloy being strengthened is substantially prohibited during the cryogenic, explosive shocking. Thus, the explosive shock is applied to one surface of the alloy while the remaining surfaces abut against a mandrel or the like, so that the entire specimen is retained to prevent deformation or stretching on a macroscopic scale.

As pointed out above, prior art cryogenic stretching processes require macroscopic deformation of the article being strengthened. The deformation in these processes usually amounts to about 5 to l0 percent or greater stretching of the alloy product. By way of contrast, the macroscopic deformation produced in the present process is kept as low as possible. Thus, stainless steel articles being strengthened by this process are generally not deformed more than 0.5 percent by the explosive shocking, and the macroscopic deformation is preferably less than about 0.1 percent. in all instances, the deformation is kept as low as possible.

FIG. 1 illustrates schematically a setup for practice of the process of this invention. In this embodiment of the invention, the entire explosive shocking equipment is maintained inside a 2-inch thick forged steel tank 10 mounted on steel base 12. Anvil 14 is also mounted on the steel base plate, and steel retainer ring 16 is bolted to anvil 14 with a number l-inch minimum diameter bolt.

Retainer ring 16 holds Cerrobend die 18 in position during the explosive shocking of test specimen 20. The Cerrobend is preferably casted around the specimen to be strengthened. Cerrobend is a eutectic alloy of bismuth, lead, tin and cadmium, produced by Cerro de Pasco Corporation of New York, New York. Since the die is used only for retention in the cryogenic, explosive process of this invention, it can be reused many times, promoting the economy of the process.

The explosive shock assembly illustrated in FIG. 1 com prises sheet explosive 22, primacord and detonator cap 24. and detonator cap leads 26 which connect to a remote detonating device (not shown). The explosive sheet is made, for example, of Type ELSOGCl Detasheet, produced by the DuPont Company, having a sheet thickness of 0.042 inch.

The alloy-strengthening setup shown in FIG. 1 also includes a copper, liquid nitrogen inlet line 28 and liquid nitrogen outlet 30 which communicate with shock specimen 20 and allow it to be cooled to cryogenic temperatures by a continuing supply of liquid nitrogen, or other suitable cryogenic medium. The liquid nitrogen is supplied from a suitable storage vessel (not shown).

The shock specimen 20, and the remainder of the assembly are held in place during the explosive shock by a large quantity of covering material 32. This covering material is preferably sand, but may be any other material suitable for use in cryogenic temperature operation, such as granular metal particles or the like. Granular covering layer 32 is retained in place by plywood retainer walls 34 and plywood baseboard 36. Slider plate 38 is provided with a plurality of thermocou;

ples which are used to determine when the shock specimen reaches the desired cryogenic temperature.

A buffer plate 40 is provided between explosive charge 22 and shock specimen 20. This butter plate may be metallic, but it preferably comprises a coating of a suitable plastic or epoxy material directly on the shock specimen. Suitable plastic and epoxy materials for this buffer plate include self-curing, synthetic neoprene rubber and cured urethane resins. Exemplary of such materials are PWA580 Neoprene rubber, produced by United Aircraft Corp., and MP950 cured urethane resin, produced by United Elastic Corp.

In operation of the test setup illustrated in FIG. 1, the Cerrobend die is cast around the shock specimen 20, and the cast composite is seated in the frame formed by steel retainer ring l6 and anvil 14. A temperature sensing plate (not shown), which operates in conjunction with the thermocouples of slider plate 38, is positioned between slider plate 38 and buffer 40, in sliding relationship to sheet explosive 22. Liquid nitrogen is then supplied to the shock specimen from a remote source through copper tubing 28, and passes over the shock specimen and out of the shock cavity through outlet 30 until the desired temperature of the shock specimen (preferably about-100 F.) is reached.

The temperature sensing plate is withdrawn from the proximity of the shock specimen by sliding sheet explosive 22 into the position shown in FIG. 1. The explosive is then detonated by actuation of the explosive assembly, thereby subjecting shock specimen 20 to explosive shock pressures of about 150 to 450 kb.

The Cerrobend die surrounding the shock specimen prevents deformation of the specimen on a macroscopic scale. There is, however, submicroscopic deformation on an extremely localized and uniform scale in all parts of the shock specimen. This can be seen by comparison of FIGS. 2 and 3 which are photomicrographs of shock specimens taken before (FIG. 2) and after (FIG. 3) cryogenic, explosive shocking in accordance with the process of this invention. The specimens of FIG. 3 have been subjected to a single explosive shock at a temperature of -100 F. and an explosive shock pressure of 250 kb.

FIG. 2 shows photomicrographs of an annealed AISI 301 stainless steel, prior to treatment by the cryogenic shocking procedure of this invention. The alloy specimen is etched with oxalic acid and then photographed under 50X and 200x magnification. The recrystallized structure shown in these photographs contains some annealing twins and occasional grain boundary carbide. It is primarily austenite containing partial dislocations with stacking faulted areas.

FIG. 3 shows the AISI 301 stainless steel alloy sample of FIG. 2 after it is subjected to cryogenic shocking in accordance with the present process. Again the sample is photomicrographed under 50X and 200X magnification. The grain size of the alloy is substantially the same as that of the annealed material of FIG. 2, but it is highly worked. About 80 to 85 percent of the structure contains martensitic needles with retained austenite as a background phase, and substantial dislocation tangles. The remainder of the structure comprises e-bands of martensite in a matrix of retained austenite. The structure shown in FIG. 3 is produced by a single explosive shock at cryogenic temperatures. Photomicrographs of double shocked stainless steel alloy specimens are generally similar to FIG. 3 except that the double shocked specimens exhibit less retained austenite and more martensitic needles.

The exact mechanism by which the strengthening of the present process is achieved is not completely understood. Without being limited to any particular theory, it is believed that the formation of martensite on cryogenic shocking is a primary strengthening mechanism. It is also believed, however, that the concentration of each phase present in the shocked alloy specimens, the size of the phase particles, and the orientation of the phase particles all correlate to the degree of strengthening achieved by this process. Thus, for example, the presence of complex dislocation networks in the cryogenically shocked stainless steels treated in accordance with the present process are believed to significantly contribute to the strengthening it achieves in these alloys.

The present process can be used to strengthen stainless steel and titanium alloys in the form of flat plates, complex geometrical configurations, castings, welded structures, and largegrain-size forgings, extrusions or other hot or warm worked structures. I

Prior art cryogenic stretching processes have generally been limited to containers which can enclose the cryogenic material used for cooling. The present process is particularly advantageous in that it can be applied to flat plates and shaped articles in addition to container-type articles.

Because the present process strengthens without requiring deformation, a pressure vessel can be fabricated and welded by conventional methods of material in an annealed condition. The vessel can be machined, if desired, in this annealed condition to the final desired size of the vessel and then strengthened by cryogenic shocking. These features of the invention, and the fact that the retention die can be reused, provide important practical and cost advantages to the present process over prior art cryogenic stretching techniques, even in application to the production of pressure vessels and containers.

The present process is particularly important in its application to the strengthening of stainless steel and titanium alloy castings. Casting is the first and least expensive step in the formation of many metal articles. In conventional metal forming operations, it is necessary to go through the steps of forging, rolling, machining, and the like after preparation of the initial casting. Through the use of the process of this invention, a casting can be directly strengthened by cryogenic explosive shocking. This technique allows many articles to be cast, strengthened, and then directly used, avoiding the expensive and time-consuming steps of forging, rolling, machining and the like.

The availability of the present process will, therefore, open a whole new field of use for castings, markedly reducing the cost of modifying cast articles to render them suitable for desired end use.

For a better understanding of this invention, specific examples illustrating it are set forth below. It is to be understood that these examples are illustrative only and are not intended to be and should not be understood as limiting the scope and underlying principles ofthis invention in any way.

EXAMPLES 1-22 These examples illustrate the application of the cryogenic shocking procedure of this invention to flat plates of AISI 30] stainless steel. This steel has the nominal composition Fe l6 to 18 Cr-6 to 8 Ni--2.0 Mnl .0 Si. The 22 specimens tested are shocked at various temperatures between room temperature and 320 F. at shock-pressures varying between I50 kb. and 450 kb. by single and multiple shock procedures. The samples are then tested to determine their strength and ductility properties.

The specimens tested in examples 1 and 2 are unshocked, control specimens for comparison purposes. The specimen of example 2 is soaked for 1 hour in liquid nitrogen to reduce its temperature to 320 F., but it is not shocked.

The results of these tests are reported in table I, below. This table shows the shock temperature and shock pressure used in each example, together with the number of explosive shocks to which each specimen is subjected. The table reports the room temperature, longitudinal strength and ductility properties of the resulting specimens including the yield strength, ultimate tensile strength, elongation and reduction area.

The procedure used in shocking is that described above and schematically illustrated in FIG. 1.

The specimens are tested in these examples in both smooth and notched condition. The notch strength of metal specimens is vary important since every scrape or bump to.a metal surface creates a notch defect. The notch sensitivity ratio (NSR) is the ratio of the strength of a notched specimen to the strength of the same specimen in unnotched form. National Aeronautical and Space Administration requirements presently consider an NSR of 0.8 acceptable, but an NSR of 1.0 or greater must be exhibited before a material is removed from the notch sensitive category.

The data reported in table I illustrates that shocking at cryogenic temperatures of about 100 to 320 F, greatly enhances the strength of 301 stainless steel alloy specimens. This data indicates that shock pressures of about 150 kb. to 450 kb. can be used, but explosive shock wave pressures of about 250 kb, are particularly effective.

invention exhibited much iiighi' strength properties, and much better strength retention, than the specimen shocked at room temperature.

TABLE II.400 F. STRENGTH PROPERTIES OF EXPLOSIVELY SHOCKED AISI 301 STAINLESS STEEL Smooth tensile specimen Notched tensile specimen Notch N0. Shock Shock sensi- Example of temp, pres, Y.S,, U.T.S., EL, R.A., U.'1.S,, EL, R.A., tlvity No. shocks F. kb. p.s.i. p.s.i. percent percent p.s.i. percent percent ratio The data also shows that two sequential shocks produce 25 Table in, below, illustrates the increased hardness of the more effective strengthening than a single explosive shock. shock specimens of examples 3-22, which are treated in ac- While the ultimate tensile strength of the test specimens varies cordance with the process of this invention. This table also ilfairly uniformly with the shock pressure used, the yield lustrates the retention of this increased hardness after strength of the materials behaves in a more unpredictable mechanical testing at 400 F. The specimens are again heated manner. Yield strengths above the 200,001 p.s.i. level are E forabout 30111111111135 before testing achieved consistently, however, by double shocking at pressures of 250 kb. and temperatures between l00 and -320 The cryogenic in process of this invention also increases the transverse strength properties of explosively shocked 301 stainless steel test specimens. The increased transverse strength properties produced in both smooth and notched specimens of various of examples 3-22 are illustrated by the results of tests reported in table IV below.

EXAMPLES 23-26 The tests of these examples are carried out to determine the effects of cryogenic cooling and shocking on the austenite to martensite transformation, and on the strengthening of A181 301 stainless steel. The treatment applied to each sample, the

TAB LE I.ROOM TEMPERATURE STRENGTH PROPERTIES OF EXPLOSIVELY SHOCKED A181 301 STAINLESS STEEL Smooth tensile specimen 1 N otchcd tensile specimen 1 Notch No. Shock Shock sensioi' temp., pres, Y.S. U.T.S. Ell, RAJ, U.T.S., EL, R.A., tivlty shocks F. kb. p.s.i. p.s.i. percent percent p.s.i. percent percent ratio 42, 500 1 RT 150 116, 500 154, 600 1 RT 250 98, 500 117, 800 5 1 250 76, 000 137, 5 1 -100 150 121, 000 150, 800 3 1 %&J 250 156, 000 172, 700 3 RT 250 1 460 350 89, 100 151, 000 1 1 000 1 i 155, 200 105, 500 l 1153 116, 000 161, 400 151, 500

5 2 ggg 3 214, 000 232, 000

50 2 g% 35 8 192, 200 200, 000 2 RT 250 192, 500 200, 000 1 -320 350 ggg 1 -a20 s50 1 320 450 136, 700 183, 200 12. 5 42 195, 500

1 Tested in longitudinal direction.

tensile strength. 5 El.=e1ongation. 6 R.A.=reduction area.

2 Soaked for one hour in liquid nitrogen.

3 Y.S.=yield strength. 4 U.T,S.=ultirnate Sh k Sh k Hardness (Re) 0c N o. of temp., pressur As After +400 F. shocks F. kb. shocked mech. test 1 RT 150 33.8 1 RT 250 l 65 250 l 100 150 1 100 250 100 250 2 250 1 388 43. 5 45. 5 2 RT 250 1 260 350 l 320 150 l 320 150 34. 5 320 150 1 320 150 l 50.8 l 320 250 1 g% 250 I 38.5

250 2 I ggg s 49- 5 2 l 250 48.7 48.7 2 320 250 RT 250 1 320 350 l 320 450 38. 5

TABLTIT T ROOM TEMPERATURE TRANSVE RSE STRENGTH P The shocked sample is then aged at 900 F. for 8 hours. The yield strength, and ultimate tensile strength are evaluated at each stage of the process, and the ductility (percent elongation) is measured following each aging step. All of these properties are measured at room temperature. The results of these tests are reported in table Vl below.

attainable in the AM 362 alloy under optimum aging conditions is 182 Ks.i., but this strength can be raised to 209 Ks.i. by cryogenic shocking in accordance with the process of this invention. It is believed that this additional strengthening is aofit'rfizs OF expLosivnLi' suoofiap AISI 301 STAINLESS STEEL Smooth tensile specimen 1 Notched tensile specimen l Shock No. of temp, shocks F.

Shock pres., Y.S.,

Example Ne. p.s.i.

no NHMHHH-H 1 Tested in transverse direction. com position df tlEr uTting sampTeT in percent austenite and percent martensite, and the yield strength of the treated sample are set forth in table V below.

In example 23, the specimen is cooled for 1 hour in liquid nitrogen, but not subjected to any explosive shock.

In examples 24-26 the samples are shocked at cryogenic temperatures in accordance with the procedures described above and illustrated schematically in FIG. 1.

This example illustrates the use of the cryogenic process of this invention in the strengthening of precipitation hardening stainless steels. The sample tested is a flat plate of AM 362 stainless steel. This steel has the composition: Fe- 1 5 Cr6.5 Ni0.8 Ti0.03 C and contains about 20 to 31 percent metastable austenite at room temperature. The sample is first annealed, and then aged for 8 hours at 900 F. This procedure is considered to introduce optimum age strengthening in the sample. Another annealed sample is then subjected to two explosive shocks to subject it to a shock pressure of 250 kb. on each occasion, with each shock being carried out at a cryogenic temperature of 100 F.

p.s.i.

El., percent R.A., percent U.T.S., EL, RA. p.s.i. percent percent EXAMPLE 28 In this example the metastable, beta-phase titanium alloy Ti-l3 Vll Cr3 Al is subjected to a double shocking procedure in accordance with the process of this invention. The first shock is carried out at room temperature and applies shock wave pressure of 250 kb. to the titanium alloy. The second shock is carried out at 200 F. and also applies a shock wave pressure of 250 kb. The room temperature yield strength of the alloy is increased from Ks.i. before shocking to 162 Ksi. after shocking, an increase of about 25v percent. The room temperature notch sensitivity ratio (N SR) of the titanium alloy is increased from l.05 before shocking to l.l8 after shocking. The room temperature ductility drops from 16 percent elongation before shocking to 5 percent elongation after shocking, but remains at a useful level.

EXAMPLE 29 The cryogenic shocking procedure of example 28 is repeated in this example, except that the titanium alloy being strengthened is Til5.5 V2.5 Al Substantial improvements are achieved in the strength properties of this alloy.

EXAMPLES 30-34 In these examples a comparison is made of the yield strength and notch sensitivity ratio of A151 301 stainless steel which has been strengthened by:

(30) conventional cold rolling,

(3 l) cryostretching, and

(32-34) cryogenic shocking in accordance with the present process.

The samples of example 30 are work hardened by cold rolling to a 42 percent increase in area. The samples of example 31 are cryogenically stretched at -320 F, to increase their area by 14.8 percent. The samples of example 32 are subjected to a single cryogenic shock of 250 kb. in accordance with the process of this invention at 320 F. The samples of example 33 are subjected to two sequential explosive shocks of 250 kb. pressure in accordance with the process of this invention at a temperature of 320 F. The samples of example 34 are subjected to two sequential explosive shocks at a shock pressure of 250 kb. and a shock temperature of ll F.

The yield strength and notch sensitivity ratio for each of these samples is determined at room temperature and these results are reported in table Vll below. These results reveal that the cryogenic shocking process of this invention achieves yield strengths and notch sensitivity ratios comparable to or greater than prior art cold rolling and cryogenic stretching processes, while avoiding the problems incident to the need for deformation of articles being strengthened by these prior Thus, in accordance with the present invention, it is possible to harden and strengthen stainless steel and titanium alloys of the type described herein while retaining satisfactory levels of ductility in these alloys. it is possible to achieve this strengthening while requiring the metal to undergo little or substantially no macroscopic defonnation; and it is possible to carry out this process in a less expensive, less time consuming, and more controllably-reproducible manner than is possible using prior art cold rolling or cryogenic stretching techniques.

The advantages of the present process can be applied to the strengthening of flat plates, shaped articles, castings, forgings, and both the welded and unwelded areas of stainless steel and titanium workpieces being strengthened.

The instant invention in its broader aspects is not limited to the specific details shown and described, but departures may be made from such details without departing from the principles. of the invention and without sacrificing its chief advantages.

What is claimed is: l. A process for strengthening metal alloys which comprises subjecting an alloy selected from the group consisting of stainless steels containing at least 20 percent of a metastable, austenitic phase at room temperature and titanium alloys consisting primarily of metastable beta-phase at room temperature to at least one explosive shock which applies a shock pressure of between about 150 and 450 kb. to the alloy at a cryogenic temperature between about --75 and 320 F., while substantially prohibiting macroscopic deformation of the alloy.

2. The process of claim 1, in which the alloy is shocked at a temperature between about 1 00 and 200 F.

3. The process of claim 1, wherein the alloy is subjected to a single explosive shock which applies a pressure of about 200 to 450 kb. to the alloy.

4. The process of claim 1, in which the alloy is subjected to multiple explosive shocks, each of which applies a pressure between about 150 and 450 kb. to the alloy.

5. The process of claim 4, in which the alloy is subjected to two explosive shocks, each of which applies a pressure between about 200 and 450 kb. to the alloy at a temperature between aboutand 320 F.

6. The process of claim 5, in which the alloy is subjected in each shock to a pressure of between about 225 and 275 kb. at a temperature of about 1 00 to 200 F.

7. The process of claim 1, in which the alloy being strengthened is a stainless steel alloy containing at least 20 percent of metastable, austenite phase at room temperature.

8. The process of claim 7, in which the stainless steel alloy consists essentially of Fel6 to 18 Cr-6 to 8 Ni-up to 2 Mn-up to 1 Si.

9. The process of claim 7, in which the stainless steel alloy being strengthened is a precipitation hardening stainless steel.

10. The process of claim 9, in which the stainless steel alloy consists essentially of Fe 1 5 Cr-6.5 Ni0.8 Ti-0.03 C.

11. The process of claim 9, in which the stainless steel alloy consists essentially of Fel7 Cr-7 Nil.l5 Al-0.7 Mn 0.4 Si0.07 C.

12. The process of claim 1, in which the alloy being strengthened is a Ti-base alloy which consists primarily of metastable beta-phase at room temperature.

13. The process of claim 12, in which the Ti-base alloy consists essentially ofTil 5 to 16 V2.5 Al.

14. The process of claim 12, in which the Ti-base alloy consists essentially ofTi-l3 V-l l Cr-3 Al.

15. The process of claim 1, in which the alloy being strengthened is in the form of a flat plate.

16. The process of claim 1, in which the alloy being strengthened is in the form of a casting.

17. The process of claim 1, in which the alloy being strengthened is in the form of a welded structure.

18. The process of claim 1, in which the alloy being strengthened is in the form of a large grain size forging, or extrusion. 

2. The process of claim 1, in which the alloy is shocked at a temperature between about -100* and -200* F.
 3. The process of claim 1, wherein the alloy is subjected to a single explosive shock which applies a pressure of about 200 to 450 kb. to the alloy.
 4. The process of claim 1, in which the alloy is subjected to multiple explosive shocks, each of which applies a pressure between about 150 and 450 kb. to the alloy.
 5. The process of claim 4, in which the alloy is subjected to two explosive shocks, each of which applies a pressure between about 200 and 450 kb. to the alloy at a temperature between about -100* and -320* F.
 6. The process of claim 5, in which the alloy is subjected in each shock to a pressure of between about 225 and 275 kb. at a temperature of about -100* to 200* F.
 7. The process of claim 1, in which the alloy being strengthened is a stainless steel alloy containing at least 20 percent of metastable, austenite phase at room temperature.
 8. The process of claim 7, in which the stainless steel alloy consists essentially of Fe-16 to 18 Cr-6 to 8 Ni-up to 2 Mn-up to 1 Si.
 9. The process of claim 7, in which the stainless steel alloy being strengthened is a precipitation hardening stainless steel.
 10. The process of claim 9, in which the stainless steel alloy consists essentially of Fe-15 Cr-6.5 Ni-0.8 Ti-0.03 C.
 11. The process of claim 9, in which the stainless steel alloy consists essentially of Fe-17 Cr-7 Ni-1.15 Al-0.7 Mn-0.4 Si-0.07 C.
 12. The process of claim 1, in which the alloy being strengthened is a Ti-base alloy which consists primarily of metastable beta-phase at room temperature.
 13. The process of claim 12, in which the Ti-base alloy consists essentially of Ti-15 to 16 V-2.5 Al.
 14. The process of claim 12, in which the Ti-base alloy consists essentially Of Ti-13 V-11 Cr-3 Al.
 15. The process of claim 1, in which the alloy being strengthened is in the form of a flat plate.
 16. The process of claim 1, in which the alloy being strengthened is in the form of a casting.
 17. The process of claim 1, in which the alloy being strengthened is in the form of a welded structure.
 18. The process of claim 1, in which the alloy being strengthened is in the form of a large grain size forging, or extrusion. 